Here are some examples of how citations and bibliographic entries would look as formatted in two of the most common documentation styles. Let's use the Smithsonian article about the monkeys who steal food.
In-text parenthetical references (or "parenthetical citations")
According to the study, some monkeys act as if a human thinks like a monkey. If they do, then monkeys exhibit "theory-of-mind," which is "the ability to impute thought and intentions to another individual, one of the cornerstones of human cognition" (Adler 60).
If they do, then monkeys exhibit "theory-of-mind," which is "the ability to impute thought and intentions to another individual, one of the cornerstones of human cognition" (Adler, 2008, p. 60).
[If this passage paraphrased the original source rather than quoting it, the page number would be omitted and the citation would be (Adler, 2008).]
Adler, Jerry. "Thinking Like a Monkey." Smithsonian 38.10 (2008): 58–62. Print.
Adler, J. (2008). Thinking Like a Monkey. Smithsonian, 38(10), 58–62.
Certain documentation styles always use footnotes or endnotes, not parenthetical citations. For example, in Chicago style, a quoted passage or a paraphrased material would be cited in a footnote, like this:
Recently, researchers have studied the intelligence of monkeys through the monkeys' ability to understand that other creatures think. Because rhesus monkeys are known to be skillful food thieves, Laurie Santos and her students used the monkeys' bad habit to find out if a monkey tries to imagine what a person is thinking in order to try to steal that person's food. According to the study, some monkeys act as if a human thinks like a monkey.1
 Jerry Adler, "Thinking Like a Monkey," Smithsonian 38 (2008): 60.
For more information on documentation styles, including links to web resources with helpful examples and guidelines for each style, see Geisel Library's webpage on Citing Sources.